This invention relates generally to a drill screw, and more particularly to a drill screw of highly corrosive resistant stainless steel material.
The ever increasing demand and requirements for corrosive resistant materials, particularly in fasteners, has highlighted a problem in the area of self-drilling screws. A typical self-drilling and tapping screw should be of a hardness of Rc 50 or above in order to successfully drill and tap in material, such as mild carbon steel. Heretofore, the corrosive resistance of the fastener had to be sacrificed in order to produce a fastener which could successfully drill and tap such materials. For example, 410 series stainless steel is a hardenable martensitic material and typically includes a chromium content of only 11.5%-13.5%. The hardening is usually accomplished by a typical heat treating or case hardening technique. However, in a 300 series stainless steel, which inludes a higher chromium content, such a structure is usually a nonhardenable, austenitic material. Such material is highly beneficial as a corrosive resistant and durable fastener but, to date, has not been capable of successfully drilling into carbon steel or similar materials.
Special steels have been developed, such as an 18--18 chromium manganese steel in order to produce a highly corrosive resistant material which is capable of drilling. Such steels, however, do have drawbacks in that they are extremely hard and, thus, are also very difficult to head and thus create a very short tool life for heading dies or the like.
Compromise solutions to this problem have been suggested in the way of a composite fastener. For example, a 300 series overcap has been suggested for use with a typically cold headed carbon steel screw. This, obviously, is capable of drilling and retaining certain corrosive resistance at the head. However, corrosion of the shank remains a problem, as well as the increased costs due to a plurality of parts, handling and the like.
Composite screws which utilize a shank and head of a 300 series steel in a point region of carbon steel which is in some manner secured to the shank has also been suggested. However, an increased handling problem obviously exists and to date no successful solution as to proper securement of the point to the shank has been suggested.
The proper combination of ease of manufacture, corrosive resistance level, and capability of drilling through carbon steel has not been successfully obtained in any of the prior art attempts thus far.